Method of retanning of chrome leather



Patented Jan. 30, 1934 METHOD OF TANNING OF CHROME Thomas Blaokadder, St. Davids, Pa, assigninto Bn'ihm & Haas Company, Philadelphia, Pa.

No Drawing. Application December 10, 193

' Serial N0. 501,477

The two principal current methods of converting raw pelt into leather are: (a) vegetable tannage (b) chrome tannage. Vegetable tannage imparts more weight to the skin, but the leather is not resistant to hot water. Chrome tannage confers resistance to hot water, and in addition the process is more rapid than with vegetable tannage. Chrome tannage by itself however gives an empty-feeling leather, and a supplementary l0 subsequent tannage with vegetable taming materials is usually given. The degree of this subsequent tannage may vary from a mild retannage with from- '2 to 5 percent of a vegetable tanning malll terial for the purpose of mordanting, up to a thorough retannage with as high as 50% of vegetable tanning material. These percentages are based on the weight of the skins.

In applying the supplementary tannage it is common practice to neutralize the leather prior to the application of the vegetable tannage. This neutralization is ordinarily carried out in a direct manner by bringing the leather in intimate contact with suitable amounts of alkali by drumming or paddling in solutions of an alkali. The neutralizing may also be carried out indirectly, as for instance when the subsequent tannage is carried out in vats where the proportion of vegetable tanning liquor is high inrelation to the amount so of leather being retanned. In such a case the vegetable tan liquor, containing as it does a considerable proportion of .the salts of weak acids, buflers the relatively strong acids present in the chrome tanned leather. In whatsoever manner 86 the neutralization is effected it is essential to give the supplementary tannage at a high pH, in order to avoid producing a harsh, tender or unevenly retanned leather. In practice, therefore, the .leather is neutralized as much as is safe, the limit being determined by the'appearance of a rough or "drawn grain. Too much neutralization gives a rough grained, spongy leather. Thus the safe working range for a successful retannage with vegetable materials is narrow, and care 48 must always be exercised or the leather will suffer from retannage in too acid a condition, or from over-neutralization.

Chrome leather may be given a supplementary tannage with synthetic tanning agents from the 80 class consisting of condensed aromatic sulfonic acids, aromatic sulfonic acids of high molecular weight, and resins peptized in aromatic sulfonic acids. The surprising feature of my invention is the discovery that this supplementary tannage may be carried out without a prior neutralization in Claim. (cl. 14H) of the leather. This feature is an important step forward in the art, as it enables the production of a retanned or mordanted chrome leather without any danger of affecting the leather adversely by the neutralization process.

The chrome leather may be taken as it comes from the tanning drums or paddles, and after washing with water to remove the free excess chrome salts is given a supplementary tannage with these synthetic tanning agents. The resuiting leather has a smoother grain and a stronger fibre than a leather retanned with a vegetable tanning material. For the purpose of obtaining the smoothest grain and strongest fibre it is preferable to take the leather in a quite acid condition. Inasmuch as some chrome leathers come from the tanning operation less acid than others it is advisable in some cases to add acid prior to this supplementary tannage. This advisability may be simply determined by dropping a few drops of suitable indicator on a cut surface of the leather and noting the apparent pH'value by the color assumed by the indicator. I prefer to carry out the supplementary tannage on leather which shows an apparent pH of less than 5. In cases where the most acid conditions are desired, the supplementary tannage may be given immediately subsequent to the chrome tannage in, the spent tanning liquor without removing the leather from the tanning drum. The supplementary tannage may vary in amount from: 2% to 50% of the synthetic tanning materials, based on the weight of the skins and is carried out in the regular manner in regular apparatus. The leather is washed after the retannage and then colored, fatliquored and finished as usual.

The following examples serve to illustrate how I carry out my invention, but my invention is not limited to these examples. It is further expressly understood that the synthetic tanning material is one of the class consisting of:

(a) Condensed. aromatic sulfonic acids, such as phenol sulfonic acid condensed with formaldehyde, acetaldehyde, .benzaldehyde, acetone, cyclo-. hexanone; naphthalene sulfonic acid condensed with these aldehydes or ketones; materials similar to the above but derived from homologues of phenol or naphthalene.

(b) Aromatic sulfonic acids of high molecular weight, such as phenanthrene monosulfonic acid; substituted naphthalene sulfonic acids like benzylor dibenzyl-naphthalene sulfonic acids? carbazole sulfonic acid; tetralin suli'onic acid.

(c) Aromatic sulfonic acids containing dispersed resins such as benzyl-naphthalene sulfonic no regular manner.

acid or tetralin sulfonic acid containing peptized phenol-formaldehyde resin.

Example 1 400 lbs. of calfskins, tanned by any customary one-bath chrome process are taken from the tannage, shaved and then washed in a drum for 10 minuteswith water at 70 to F. A cut made in the neck of the wet leather should at this stage show a yellow to green coloration when wet with a 1 6% solution of brom cresol green,

indicating an apparent pH value of the stock between 4 and 4 units. The skins are then drummed with 50 gallons of water and 40 lbs. of a synthetic tanning material prepared as follows:

3.7 lbs. of phenol 4.2 lbs. of sulphuric acid (66 Beaum) are mixed and heated at 100 for 2 hours. The water soluble mixture is cooled to 30 C. and diluted with 1.1 lbs. water. 1.7 lbs. of formaldehyde (37%) is added slowly with good stirring and cooling, and the stirring continued for 5 hours thereafter at a temperature of 25-30 C. The mixture is diluted somewhat and 1.1 lbs. of caustic soda added in rather strong solution while cooling. The resulting solution is vdiluted to 40 lbs.

The drumming is continued for 30 minutes,

whereupon the skins are washed for a further- 30 minutes, with a good stream of water passing through the drum. The skins may then be colored with direct or acid dyes, topped if desired with basic dyes, or dyed entirely with basic dyes, fatliquored and finished in the regular manner.

Example 2 400 lbs. of kidskins tanned by a regular twobath chrome process are taken from the hypo reduction bath and washed to free them from excess hypo, then shaved and washed 10 minutes in order to wash them free of adhering shavings. A cut is made in the neck of several skins and the apparent pH value estimated by droppin a solution of brom cresol green on the cuts. If this indicator indicates a pH value of over 4.5 units, but not otherwise, I add 4 lbs. muriatic acid 20 previously diluted in abucket of water to the leather in the drum with 50 gallons of water, and drum for 15 minutes. I then add 25 lbs. of a synthetic tanning agent prepared as below, and drum a further 20 minutes.

2.85 lbs. of 4-4'-dihydroxydiphenyldimethyl methane (Beilstein' Band V1 p. 1011) are sulphonated by heating with 2.5 lbs. of cone. sulphuric acid at 70 C. for 5 hours. The excess of sulphuric acid is neutralized with caustic soda and the solution diluted to 25 lbs.

The leather is then washed in the same drum for 30 minutes with a good stream of water passing through the drum, and the coloring, tatliquoring and drying of the leather follow in the Example 3 drummed with this acid solution for 1 hour,

running, 11 lbs. or chromic oxide in the form of sumcient. The leather lies. in this acid/liquor overnight, or about 16 hours and is again drummed 10 minutes. Thereupon 50 lbs, of a synthetic tanning material prepared as described below is added while the drum is running, and the drum run a further period of 30 minutes. 12.8 lbs. naphthalene are sulphonated by heating with 15.0 lbs. sulphuric acid 98% until all is completely soluble in water. The solution is I cooled, diluted with water, and 16.2 lbs. formaldehyde 37% aqueous solution added. The mixture is heated at C. until condensation is com plete, then diluted with water, 4 lbs. caustic soda added to'neutralize the excess of mineral acid, and the final weight adjusted by addition of water to give 130 lbs. After treatment with this solution the leather is washed, and when the apparent pH value has risen to between 4.5 and 5.0 units it is given a thorough retannage with liquid quebracho extract. This operation may be carried out in the same drum or in a paddle. or by hanging the sides in a vat of quebracho liquor.

' Example 4 y800 lbs. of chrome side leather tanned by a regular one-bath chrome process and retanned with a further amount of chrome in the regular manner are washed 10 minutes and a few drops of a 1 6% solution of brom cresol green dropped into a out. If this indicator indicates by a blue color an apparent pH value of over 4.5 units I add to the leather and water in the drum sufficient sulphuric acid, previously diluted in a bucket of water, to reduce the apparent pH value of the stock to 3.5 units. The leather is whereupon I add 80 lbs. of a synthetic tanning material prepared as follows: 4

A mixture oi 9 lbs. of naphthalene 9 lbs. of sulphuric acid 98% is heated to 130 C. for 6 hours, then cooled. to 100 C. Then 21 lbs. of dibenzylnaphthalene 9.7 lbs. of sulphuric acid 70% are added.

Temperature is adjusted to 85 and 7.2 lbs. of formaldehyde 37% are run in over a period of 2 hours, while the mixture is well stirred. Stirring is continued .at 85-90 C. for 4 hours. The mixture is then diluted with 90 0.0. water and the excess of mineral acid neutralized with caustic soda, given in concentrated solution. The product is diluted to 80 lbs. In this waya creamy liquid is produced which disperses readily in water.

Drumming is continued for a further period oi. 1 hour. The leather is then washed for 1 hour with a good stream of water passing through the drum, when it is ready for coloring, fatliquoring and finished in the regular manner.

Example 5 600 lbs. of pickled sheepskinsai'e put in a drum with-12 gallons of a 5% salt brine and drummed 5 minutes. While the drum is still a basic chromium sulfate of 65% acidity are added in two feeds, one hour apart. One hour after the second feeding, while the apparent pH value of the skins is below 3 units, 42 lbs. of a synthetic tanning material prepared as described be- 45 low are added and the drum run a further 30 minutes.

12 lbs. tetrahydronaphthalene are added to 16.3 lbs. sulphuric acid 98% at 100 C. The mixture is stirred at this temperature-until all is 159 completely soluble in water, then diluted with water and completely neutralized with lime to remove excess of mineral acid. The filtered solution is reacidi'fied with 8 lbs. sulphuric acid, allowed to settle several days, and the clear liquor drawn off. The weight is adjusted by addition of water to give 100 lbs. Suflicient bicarbonate of soda is now added to raise the apparent pH value or the leather to 3% units. With the drum standing still, water is now added until the leather is well covered; the drum run 10 minutes to mix the contents thoroughly. 'Rhe following morning the leather is well washed with a good stream of water in the same drum and removed for coloring, fatliquoring and drying by the usual methods.

It is to be understood, of course, that the above examples are merely illustrative of the processes intended to be covered by this application and I am in no way confined to the methods of operation, temperatures, proportions of ingredients, etc., except as indicated in the claims which follow, since it is obvious to anyone versed in the art that wide deviations are possible without de: parting from the scope of this invention. In the above examples methods are given for preparing the tanning materials but it is to be understood that the following claims cover the materials listed at page 1, line 97 to page 2, line 2 of the patent, no matter how prepared since the method of preparation is immaterial.

It is understood that in the following claims the words apparent pH value mean the pH value as estimated by the use of 1/20% solutions of the following indicators thymol blue, bromphenol blue, brom-cresol green, brom cresol purple, when employed according to the methods described in the above examples.

What I claim is:

1. In the making of leather the step of retanning chrome leather which comprises the application of synthetic tanning materials belonging to the class consisting of condensed aromatic sulfonic acids, aromatic sulfonic acids of high molecular weight, and resins peptized in aromatic sulfonic acids, without neutralizing the leather prior to retannage.

2. In the making of leather the step as described in claim 1 in which the retanning is carried out at an apparent pH value of between 2% and 5 units.

3. In the making of leather the step of retanning chrome leather after fixation of the chrome tannage, which comprises the application of; aromatic suli'onic acids at an apparent pH value of between 2% and 5 units.

4. In the making of leather the step as described in claim 3 in which the synthetic tanning materials are aromatic sulfonic acids of high molecular weight.

5. In the making of leather the step as described in claim 3 in which the synthetic tanning materials are resins peptized in aromatic sulfonic acids.

6. In the making of leather the step of retanning chrome leather after fixation of the chrome tannage, which comprises the application at an apparent pH .of 2 to 5 units of phenol sulfonic acid condensed with an aldehyde.

7. In the making of leather the step of retanning chrome leather, after fixation of the chrome tannage,,which comprises the application at an apparent pH of 2 /2 to 5 units of phenol sulfonc acid condensed with formaldehyde.

8. In the making of leather the step of retanning chrome leather, after fixation of the chrome tannage, which comprises the application at an apparent pH of 2 to 5 units of a sulfonic acid of a phenol condensed with formaldehyde.

9. In the making of leather the step of retanning chrome leather, after fixation of the chrome tannage, which comprises the application at an apparent pH of 2 to 5 units of naphthalene sulfonic acid condensed with an aldehyde.

' 10. In. the making of leather the step of retanning chrome leather, after fixation of the chrome tannage, which comprises the application at an apparent pH of 2 to 5 units of naphthalene sulfonic acid condensed with formaldehyde.

THOMAS BLACKADDER. 

